Re: Why need single hop BFD? Does single Hop BFD requires to travese all possoble links beween the two neighbors?

[sadasgup <sadasgup at cisco.com>]

Title: Re: Why need single hop BFD? Does single Hop BFD requires to travese all possoble links beween the two neighbors?

>>Any physical media, like 802.3, SONET, DWDM wavelength all have physical failure indication. Each neighbor can also use the physical failure indication to declare the >>connectivity between two immediate neighbors, which is much faster than a BFD session, isn’t it? It also needs less processing on the router/LSR, there won’t be any >>proactive periodical sending BFD over the link anymore.  
 
>>Can you explain (or add to the document) what is the reason for having single hop BFD? 
>>Is single Hop BFD only for the Tunnel scenario? 

Linda,

Have you considered scenarios where two routers are connected via a Ethernet switch or equivalent? Something like this -

Router A -------L2 Switch-------Router B

If there is a problem on one side of the connectivity (e.g. Router A to L2 Switch), the switch will hide this failure from Router B. This will impact the IGP convergence, since  Router B has no indicatation of failure at the physical layer.

Regards,
Santanu



On 08/11/09 6:40 PM, "Linda Dunbar" <ldunbar at huawei.com> wrote:

> Dave and David,
>  
> Forgive me for not aware of the history of the BFD development. Reading through the “BFD for IPv4 and IPv6 (Single Hop)” (draft-ietf-bfd-v4v6-1hop-10.txt), I am not clear why single hop BFD is needed, especially for two immediately connected neighbors. There is Hello message between two immediate neighbors. If two immediate neighbors need logical layer to detect any failure between the two immediate neighbors, they can use the Hello message to achieve this purpose. Even though Hello message is from control Plane, it would be much less work for routers/LSRs to monitor the Hello messages than creating a new BFD session. 
>  
> Any physical media, like 802.3, SONET, DWDM wavelength all have physical failure indication. Each neighbor can also use the physical failure indication to declare the connectivity between two immediate neighbors, which is much faster than a BFD session, isn’t it? It also needs less processing on the router/LSR, there won’t be any proactive periodical sending BFD over the link anymore.  
>  
> Can you explain (or add to the document) what is the reason for having single hop BFD? 
> Is single Hop BFD only for the Tunnel scenario? 
>  
> In Section 2 (Application and Limitation), the last paragraph does indicate that the transmitted packets are immediately routed back towards the sender on the interface over which they where sent if BFD Echo function is used. But when Link Aggregation is used to bundle the multiple parallel links between two neighbors, how does the network layer enforce which link to send back the “echo” message?
>  
> Even if BFD ECHO can be enforced to be sent back on the same interface port so that the individual link’s failure can be detected, what can this fault do when this fault can’t affect the connectivity between the two immediate neighbors in control plane’s view?  All the links are bundled and two immediate neighbors are still connected? 
>  
> Thank you very much, 
>  
> Linda Dunbar
>  
> Advanced Technology Dept, Wireline Networks,
> Huawei Technologies, Inc.
>